The present invention relates to a process for the production of a high yield, high strength chemithermomechanical (CTMP) pulp from hardwood and, more particularly, to a chemithermomechanical pulping process employing a two-stage chemical treatment wherein hardwood chips are sequentially treated first with an alkaline liquor and then with a sulfite liquor to chemically soften them prior to mechanical defibration.
In recent years there has been increased use of bleached hardwood chemical pulps in the manufacture of papers and, particularly, printing papers. These pulps have relatively high strength as compared to the refiner pulps. They are frequently combined with long fiber pulps to improve the printability of the finished paper.
A demand for a higher yield hardwood pulping process has accompanied the increased use of hardwood pulps.
In wood lignin, concentrated in the intercellular layer known as the middle lamella, cements the cellulose fibers to each other. The main purpose of chemical pulping processes, such as sulfite pulping, is to dissolve this layer so that the wood structure breaks down into individual fibers without the use of substantial mechanical action. This is accomplished by cooking the wood chips at high chemical concentrations and high temperatures for a prolonged period of time and results in low yields based on the dry chips because a substantial amount of the wood, particularly lignin, is removed.
The Kraft process and the sulfite pulping process are examples of chemical pulping processes in which lignin is removed from the wood. They are relatively low yield processes as they typically yield about 40 to 60% based on dry chips depending on the wood and the conditions employed. Typical acid sulfite pulping processes involve cooking the wood chips at temperatures of about 120 to 150.degree. C. and pressures in excess of one atmosphere for 5 hours or more. In alkaline sulfite chemical pulping processes, temperatures as high as 180.degree. C. are sometimes used.
One attempt which has been made to improve the yield of sulfite pulps, which is relevant from the standpoint of the present invention, involves pre-treating the chips by holding them in an alkaline solution for a period of time. Examples of such sulfite pulping processes are described in U.S. Pat. No. 3,177,110 to Ogait and U.S. Pat. No. 3,354,030 to Williams. However, these processes remain essentially chemical pulping processes which rely upon the removal of most of the lignin from the wood and do not provide high overall yields. For example, the yields illustrated in the Ogait patent only range from about 55 to 65%. By contrast, the major portion of the lignin is retained in CTMP processes due to the use of much milder conditions in the chemical treatments.
U.S. Pat. Nos. 3,479,249; 3,617,435; and 4,795,574 to Kalisch describe a semi-chemical sulfite pulping process which can be considered a hybrid of the prior chemical processes described above and the prior chemimechanical pulping processes described later.
In accordance with the teachings of the Kalisch patents, pulping is accomplished by impregnating wood chips with sodium hydroxide or sodium carbonate and, thereafter, cooking the chips in an aqueous solution of sulfur dioxide for 2 to 3 hours. In the latter treatment, the sodium hydroxide or sodium carbonate in the chips reacts with sulfur dioxide to produce bisulfite in the chips which reacts with the lignin to solubilize it in a known manner. After cooking, the chips are defibered by passing them once through a disc refiner. While the Kalisch process provides higher yields than obtained in chemical sulfite pulping processes, the yields reported in the patent are about 75% or less.
Mild sulfite treatments have been used in chemimechanical pulping processes to chemically soften the wood fibers and thereby facilitate mechanical defibration. It is known that sulfonation renders lignin more hydrophilic or less hydrophobic so that the fibers swell in the sulfite liquor and the chips become easier to defibrate. These chemimechanical processes produce higher yields than chemical or semi-chemical processes, but the pulp strength is not as good. Typically they are also more effective with softwoods than hardwoods.
U.S. Pat. No. 4,116,758 to Ford et al describes a process which is especialy suited to softwood pulps whereby wood chips are softened prior to defibration by sulfonating the lignin without substantially removing it from the wood. This is typically accomplished by maintaining the wood chips in a solution containing 50 to 180 g/l sodium sulfite at 100 to 140.degree. C. for approximately 30 to 60 minutes without an alkaline pre-treatment and is followed by disc refining. The Ford et al process is characterized in that a level of sulfonation is achieved which is at least 85% of the maximum degree of sulfonation that can be achieved without reducing the pulp yield below 90%.
Asplund, U.S. Pat. No. 3,558,428, teaches a chemimechanical pulping process employing a sulfite treatment wherein more rapid and effective impregnation of the sulfite is obtained by pre-impregnating steamed wood chips with an alkaline hydroxide solution. To avoid discoloration, this pre-impregnation step is limited to a very short duration and functions to implant hydroxide in the wood fibers without allowing it to react substantially with the hemicellulose and other wood components to swell the fibers as in other alkaline chemimechanical pulping processes. Immediately after impregnating the fibers with sodium hydroxide liquor, the chips are sluiced into a pressure vessel which is simultaneously supplied with gaseous sulfur dioxide and steam. In the pressure vessel the sodium hydroxide in the fibers rapidly reacts with the sulfur dioxide to produce bisulfite. The chips are retained in the pressure vessel for 1 to 20 minutes after which they are mechanically defibrated.
The Asplund pulping process can be classified as a single stage CTMP process because the reaction of the chemicals with the wood proceeds only after the alkali combines with sulfur dioxide. Therefore, the Asplund process must be contrasted with the teachings of this invention where the chemical treatment proceeds sequentially in two distinctly separate stages using two different chemical liquors. The conditions of chemical treatments also differ as explained later.
A chemimechanical process for treating beechwood chips wherein the chips are impregnated with a solution containing sodium hydroxide and sodium sulfite is also described in the literature by Patt et al, "Studies on the Production of Chemimechanical Pulps from Beech and Birch", Holz Roh. Werkst., 41(2), pp. 51-4, (1983). Textor, U.S. Pat. No. 3,016,324 also discloses a chemimechanical pulping process wherein wood chips may be impregnated with sodium sulfite liquor by squeezing the chips in a screwpress and spraying the chips emerging from the press with sulfite liquor and then defibrating.
Chemimechanical pulping processes in which the chips are impregnated with alkaline liquors are also known.
One proposed mechanism by which the use of alkali, such as caustic soda, increases the strength of mechanical pulps is by increasing the number of acidic groups in the wood (see S. Katz et al, "A Mechanism for the Alkali Strengthening of Mechanical Pulps", TAPPI, Vol. 64, No. 7, pp. 97-100, July, 1981). The counter-ions of these acidic groups are said to draw additional water into the cell wall and the accompanying swelling and plasticization enhance the ability of the fibers to bond and the overall strength of the pulp improves.
U.S. Pat. No. 4,187,141 to Ahrel discloses a process for producing bleached chemimechanical pulp wherein screwpressed wood chips are subjected to a two-stage impregnation with an alkaline peroxide solution in which the impregnated chips are introduced into a pressure vessel and ground between a pair of rotating discs. This process provides a pulp having high brightness but relatively low strength.
Lachenal et al, in an article entitled "Hardwood Hydrogen Peroxide Chemimechanical Pulps", TAPPI, Vol. 62, No. 5, pp. 53-57 (May, 1979) discloses a process for producing bleached hardwood hydrogen peroxide chemimechanical pulps wherein hardwood chips are impregnated with an alkaline peroxide liquor, allowed to stand for 1.5 to 2 hours at 40.degree. to 60.degree. C., and defibered by passing the impregnated chips through a Sprout-Waldron laboratory refiner at a consistency of about 5%. The chips are destructured in a cylinder press prior to impregnating the relaxed chips by mixing them with an alkaline peroxide liquor at a liquor to wood ratio of 3.5 to 1.
While chemimechanical and chemithermomechanical processes have been used in the production of hardwood pulps in the past, they have not provided high strength. Thus, there is a need for a high yield, high strength process for pulping hardwoods.